Communication systems with electromagnetic transponders are more and more frequent, particularly since the development of near-field communication (NFC) technologies.
Such systems use a radio frequency electromagnetic field with a device (terminal or reader) to communicate with another device (card).
To optimize the quality of the communication, the resonance frequency of an oscillating circuit of the device detecting a field is generally adjusted due to capacitive elements having a settable value.
In recent systems, the same NFC device may operate in card mode or in reader mode (for example, in the case of a near-field communication between two cellular phones). It is then frequent for devices to be powered with a battery and for their functions and circuits to be set to standby to avoid consuming power between periods of use.
The devices then have to be “woken up” when they are within each other's range. The field detection should however be operative in standby mode.